Electromagnetic device with embedded windings and method for its manufacture

ABSTRACT

A powdered magnetic material stator core with embedded stator windings and a method for its manufacture. Embedding the windings within a radially compacted powdered magnetic material stator core enables equivalent or better electromagnetic performance in a significantly reduced size. Radial compaction of the powdered magnetic material minimizes the distortion of the stator windings during compaction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of electromagnetic device design andmanufacturing.

2. Description of the Prior Art

An electromagnetic device, such as an electric motor or an electricgenerator, contains two electromagnetic components: a stationarycomponent known as a “stator,” and a rotating component known as a“rotor.” In the most common embodiment, the rotor and the stator arecylindrical in shape. The cylindrical rotor is installed inside thehollow, cylindrical stator in such a way that when the rotor rotates,the outer surface of the rotor is proximate to, but does not touch, theinner surface of the stator. The space between the outer surface of therotor and the inner surface of the stator is known as the “air gap.”

It is known in the art that a stator and a rotor each may bemanufactured from a core made from a magnetic material, around which orwithin which insulated electrical conductors known as “windings” areinstalled. The rotor core and stator core together form the magnetic“flux path” for the electromagnetic device.

A typical stator of a design known in the art is comprised of a hollow,cylindrical core, the inner surface of which contains slots which extendthe fall length of the core parallel to the axial direction of the core.The portions of the stator core between the slots are known as the“teeth.” The measurement made by adding the width of one slot measuredat the base of the slot to the width of one adjacent tooth is known asthe “slot pitch.”

The prior art stator windings are inserted in the slots in the core,usually by a manual means. After the stator windings are installed intothe stator core, the stator may be finished by filling the remainingvolume of the stator slots and coating the external surface of thestator with a non-reactive, non-conducting material such as, forexample, varnish or epoxy. The non-reactive, non-conducting materialserves to protect the stator from corrosion, and to prevent the statorwindings from moving within the stator slots during use. Such movement,if permitted, could damage the electrical insulation on the statorwindings, and/or could alter the electromagnetic characteristics of thestator.

It is well known in the art to manufacture a stator core from sheetsteel. Steel laminations are punched from the sheet steel. The punchedsteel laminations include slots, alignment holes, and other assemblyfeatures. The punched steel laminations are stacked so that the innersurface of the core, the outer surface of the core, the slots, and thealignment holes are aligned. The stacked steel laminations then aresecured together by methods known in the art including, for example,welding or riveting.

The method of manufacturing a stator core from sheet steel laminationspossesses several disadvantages. The process of punching the steellaminations from the sheet steel creates scrap steel pieces, which oftencannot be used productively by the manufacturer. In addition to the costof the wasted sheet steel pieces, often the manufacturer must incuradditional expense involved with the disposal of the wasted sheet steelpieces. Finally, the process of producing the finished stator core fromthe raw sheet steel is a multiple step process requiring expensivematerial handling to be performed during and/or between each processstep.

U.S. Pat. No. 4,947,065 to Ward et al. disclosed another method formanufacturing a stator core using iron powder particles coated with athermoplastic material. The method disclosed in U.S. Pat. No. 4,947,065addresses the disadvantages present in the prior art method ofmanufacturing a stator core from sheet steel laminations. Scrap iseliminated by the use of a premeasured amount of thermoplastic coatediron particles. The stator core is formed by heating the premeasuredamount of the thermoplastic coated iron particles to a predeterminedtemperature, placing the heated particles into a heated mold that isshaped to produce a stator core of the desired shape, activating a meansfor compacting the heated particles within the heated mold, therebycompacting the heated particles within the heated mold for apredetermined time at a predetermined pressure. Material handling isreduced because the raw thermoplastic coated iron particle material ismanufactured into a finished stator in fewer process steps. The statorcore of Ward et al. does not overcome all disadvantages of a prior artstator core made with steel laminations. To fabricate a finished statorfrom a stator core according to the disclosure of Ward et al., thestator windings must be installed into the slots by a manual means afterthe stator core is formed, as was required in the stator core made withsteel lamination.

Stator windings are conventionally produced from an insulated electricalconductor of types known in the art including, for example, insulatedsingle strand copper wire. The insulated electrical conductor isconventionally formed by methods known in the art into substantiallycylindrical winding configurations which will fit within the slots inthe stator core, and which will produce the desired electrical effectwhen the windings are placed in a moving magnetic field, or the desiredmagnetic effect when the windings are energized with an electriccurrent. The windings are inserted into slots in the stator to maximizethe electromagnetic coupling between the windings and the flux path, andto minimize the air gap between the rotor and stator. The portion of thewindings which is aligned parallel to the axial direction of the core isconventionally known as the “active portion” of the windings. Theportions of the windings which resides outside the stator core at eachaxial end of the stator core, and which function to conduct electricityfrom the active portion of the windings which resides in a first slot tothe active portion of the windings which resides in a second slot, areconventionally known as the “end turns.”

Electric motors and generators operate on the principle of magnetic fluxcutting. Electric motors and generators have a source of magnetic flux,such as an electromagnet or a permanent magnet, and a set of windingsthat intercept the flux. The flux path is always ferromagnetic. The fluxis cut when rotation of the rotor occurs. The desired torque and powerset the rotor dimension, while the stator dimensions are driven by boththe rotor dimension and by the flux return requirements. An importantrotor dimension is the “rotor active volume.” If “r” is the rotor radiusand “l” is the rotor active length, then the rotor active volume “X” iscalculated as “X=(πr²l).”

A vehicular alternator is an example of electromagnet based electricgenerator. In a vehicular alternator, the magnetic flux is generatedwith a multi-pole electromagnet in the rotor. It is desired in the artto maximize the average magnetic flux density, or the “magneticloading,” of the air gap. The magnetic loading may be limited bymagnetic saturation of the stator core. A disadvantage present in priorart stator design using internally slotted stator cores, is that theslots reduce the internal surface area of the stator adjacent to therotating rotor, thereby reducing the ability for magnetic flux to flowbetween the stator and the rotor. Due to the reduced internal surfacearea, the stator core teeth reach magnetic saturation more readily thanwould a stator core without internal slots. When the stator core teethsaturate, the magnetic flux density in the air gap is limited to theratio of the tooth width to slot pitch multiplied by the saturation fluxdensity of the stator material. For a typical vehicular alternatorstator material the saturation flux density is about 1.5 T, and thetooth width to slot pitch ratio is about ½, making the magnetic loadingabout 0.75 T.

Reducing the slot width and increasing the tooth width increases themagnetic loading by increasing the internal surface area of the statoradjacent to the rotating rotor. However, because the slot must carry afixed total electric current to meet the desired performancecharacteristics of the electromagnetic device, decreasing slot widthrequires an increase in slot depth to enable the slot to carry the sametotal electric current. Increasing the slot depth while maintaining thesame total electric current requires increasing the radius of thestator. It is known in the art of motor and generator design to balancemagnetic loading and stator dimension in an attempt to find the optimumsolution for each application of a particular electromagnetic device.

The magnetic loading of the stator core may be increased withoutincreasing the stator dimensions by fabricating the stator core from amaterial known in the art to have a higher saturation flux density thansteel, such as, for example, an alloy of neodymium iron boron. Suchmaterials improve magnetic loading, but at a substantially higher cost.It is desired to have a stator core fabricated from a readily available,low cost, magnetic material, wherein a higher degree of magnetic loadingmay be achieved without a corresponding increase in stator size.

Another disadvantage of the prior art stator designs arises fromlimitations on the amount of electrical current which can be carried inthe stator windings installed in a stator slot. The total currentcarried in the slot is calculated from the current carried in eachconductor multiplied by the number of conductors wound into the slot. Atypical conductor packing factor for vehicular alternator stators islimited to about 30%, which means that only 30% of the slot volume isoccupied by conductors.

It is known in the art that the total current carried in the statorslots, and therefore the conductor packing factor, is limited by theneed to dissipate the heat generated by electrical resistance in theconductors. The heat must be dissipated either through the stator core,or through the conductors themselves to the end turns of the conductors.The non-reactive, non-conducting material used to fill the stator slotssubstantially thermally isolates the conductors from the stator core. Asa result, most of the heat must be dissipated through the end turns. Thetotal current carried in the stator slots, and therefore the conductorpacking factor, can be significantly increased by providing a directcooling path through the stator core.

Another disadvantage of the prior art stator designs arises from thesignificant contribution to stator size made by the end turns of thewindings. It is known in the art of vehicular alternator design that endturns add length to a stator while serving no significant powerproducing function. The end turns are extended to facilitate electriccurrent conduction, and to serve as the heat rejection surface for thewindings. The end turns may increase the overall length of a stator by afactor of about 2.5-3 times the active length. It is known in the artthat the end turns must be enclosed within the vehicular alternatorhousing, thus the overall length of the vehicular alternator of thisdesign is increased by similar factor. A stator design wherein the endturns are contained within the volume occupied by the stator core isdesirable.

U.S. Pat. No. 5,536,985 to Ward et al. disclosed a rotor assemblywherein the rotor core is comprised of compacted soft magnetic particlescoated with a non-magnetic binder, and wherein the rotor windings areembedded within the rotor core. Three methods of manufacturing the rotorassembly are disclosed in U.S. Pat. No. 5,536,985. In the firstdisclosed method, the rotor core is manufactured by filling a die cavitywith a predetermined amount of soft magnetic particles coated with anonmagnetic binder. The soft magnetic particles within the die cavityare heated at a predetermined temperature and axially compacted for apredetermined time at a predetermined pressure to form the solid rotorcore. After the rotor core is removed from the die cavity, preformeddiscrete rotor windings are embedded into the rotor core in a circularpattern parallel to the axial direction of the core by inserting thediscrete rotor windings into preformed holes in the core.

In the second method of manufacturing the rotor assembly disclosed inU.S. Pat. No. 5,536,985, the discrete rotor windings are preformed andinserted into holding devices within a die cavity so that the rotorwindings are oriented in a circular pattern parallel to the axialdirection of the core. The die cavity then is filled with soft magneticparticles coated with a nonmagnetic binder. The soft magnetic particlesand the rotor windings within the die cavity are heated at apredetermined temperature and axially compacted for a predetermined timeat a predetermined pressure to form the solid rotor core with the rotorwindings embedded therein.

In the third method of manufacturing the rotor assembly disclosed inU.S. Pat. No. 5,536,985, the discrete rotor windings are preformed andinserted into holding devices within a two-part mold comprised of anupper mold part and a lower mold part. The holding devices are includedin the lower mold part. The lower mold part containing the rotorwindings then is filled with soft magnetic particles coated with anonmagnetic binder, and the mold is sealed by placing the upper moldpart onto the lower mold part. The sealed two-part mold containing thesoft magnetic particles and the rotor windings is placed inside aevacuated isostatic compaction chamber. The isostatic compaction chamberis sealed, and the sealed chamber is filled with hydraulic fluid heatedto a predetermined temperature, which exerts a predetermined pressureuniformly on all surfaces of the mold. After a predetermined time, thehydraulic fluid is drained from the compaction chamber. The drainedchamber is opened and the two-part mold is removed therefrom. The moldis opened destructively to reveal a rotor core with embedded rotorwindings.

The performance of an electromagnetic device may be measured by thetorque density “D” and power density “Z.” These measurements can bederived from the magnetic loading and the electrical loading of thedevice. The magnetic loading, “B,” is the average magnetic flux densityof the air gap. The electrical loading “L” is calculated by dividing thetotal current in the slots “I” by the slot pitch “p.” In equation form,it can be represented as “L=(I/p).”

To arrive at the measurements of torque density and power density,several intermediate calculations must be made. First, the tangentialforce “F” acting on the rotor may be calculated by the equation“F=(BL2πrl),” where “B” is the magnetic loading, “L” is the electricalloading, “r” is the rotor radius, and “l” is the active length of therotor.

The torque “Y” produced by the rotor may be calculated by the equation“Y=(Fr),” where “F” is the tangential force acting on the rotor, and “r”is the rotor radius. The power “P” produced by the rotor may becalculated may be calculated by the equation “P=(Fv),” where “F” is thetangential force acting on the rotor, and “v” is the rotational speed ofthe rotor multiplied by the radius of the rotor (known as the “tipspeed” of the rotor).

We can normalize the power and torque to the rotor active volume toobtain the torque density and the power density, which are directmeasures of performance of the motor or generator. The torque density“D” therefore may be calculated by the equation “D=(Y/(πr²l)),” where“Y” is torque, “r” is the rotor radius, and “l” is the active length ofthe rotor. By algebraically substituting for the variables and thenalgebraically reducing the equation, an equivalent representation of theequation can be shown as “D=(2BL),” where “B” is the magnetic loading,and “L” is the electrical loading.

The power density “Z” may be calculated by the equation “Z=(P/(πr²l)),”where “P” is power, “r” is the rotor radius, and “l” is the activelength of the rotor. By algebraically substituting for the variables andthen algebraically reducing the equation, an equivalent representationof the equation can be shown as “Z=((2BLv)/r),” where “B” is themagnetic loading, “L” is the electrical loading, “v” is the tip speed ofthe rotor, and “r” is the rotor radius.

The measurements of power density and torque density will illustrate tothose skilled in the art that magnetic loading and electrical loadingare fundamental to electromagnetic device performance. An increase ineither the magnetic loading or the electrical loading, or both, relatesdirectly to an increase in power density and torque density. Increasingthe tip speed and/or decreasing rotor radius also will increase thepower density.

It is desired to produce a stator which overcomes the disadvantagespresent in the prior art. For example, it is desired to produce a statorwhich may be manufactured economically from readily availableferromagnetic materials with a minimum of waste and a minimum ofmaterial handling, which possesses electromagnetic capabilities requiredby a variety of applications, and which embodies a smaller, moreelectrically efficient design.

SUMMARY OF THE INVENTION

The present invention is a novel powdered magnetic materialelectromagnetic device with embedded windings, and a novel method forits manufacture. In one embodiment, the electromagnetic device is astator. The windings comprise preformed insulated electrical conductors,as are common in prior art electromagnetic device designs. The windingsare placed into a compaction die cavity, and the remaining volume of thecompaction die is filled with a quantity of powdered magnetic materialwhich is sufficient to fully surround and encapsulate the windings aftercompaction. Radial compaction is used to compact the powdered magneticmaterial into a solid magnetic structure with the preformed windingsembedded therein. Radial compaction avoids or reduces distortion of thewindings during compaction. Radial compaction by the dynamic magneticcompaction method has been found to produce a suitable result. Anelectromagnetic device with embedded windings according to the presentinvention will be of sufficient density and physical strength to be asubstitute for prior art devices in a variety of applications including,for example, vehicular alternator applications. By embedding thewindings within the core, an electromagnetic device according to thepresent invention may be smaller and of a lesser mass than a prior artdevice offering similar performance.

DESCRIPTION OF THE DRAWINGS

The features and advantages of this invention, and the manner ofattaining them, will be more apparent and better understood by referenceto the following descriptions of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of one-quarter of a stator according tothe prior art.

FIG. 2 is a cross-sectional view of one-quarter of one embodiment of astator according to the present invention.

FIG. 3 is a cross-sectional view of a dynamic magnetic compactionapparatus and concept.

FIG. 4 is a top view of the dynamic magnetic compaction apparatus ofFIG. 3.

FIG. 5 is a perspective partially fragmentary view of a finished statoraccording to the present invention.

DESCRIPTION OF THE INVENTION

The present invention is a novel method for manufacturing a stator corefrom powdered magnetic material, and embedding stator windings withinthe stator core, and a novel powdered magnetic material stator core withembedded stator windings produced by the method.

To better illustrate the advantages of the present invention, it isuseful to examine a stator made according to the prior art methods. FIG.1 shows one-quarter of a cross-section of such a prior art stator. InFIG. 1, stator 10 is shown with a plurality of windings 11 installed ina plurality of slots 12. Stator core 13 is fabricated from a pluralityof stacked steel laminations (only the top lamination being shown inFIG. 1). Stator core 13 includes a plurality of teeth 14 which definethe slots 12. Windings 11 were shaped into the desired configurationprior to installation into stator core 13. During installation intostator core 13, windings 11 were inserted into slots 12 through slotopenings 15.

FIG. 2 shows one-quarter of a cross-section of one embodiment of astator according to the present invention. In FIG. 2, stator 20 is shownwith a plurality of windings 21 embedded therein. Stator core 22 of thisembodiment is fabricated from powdered magnetic material. As in theprior art, windings 21 are shaped into the desired configuration priorto being surrounded with the powdered magnetic material which comprisesstator core 22. Stator core 22 includes a plurality of slots 25 in whichwindings 21 reside. A plurality of teeth 23 separates the plurality ofslots 25. Unlike the prior art, stator core 22 has a continuous innersurface 24, with no slot openings.

In this embodiment of the present invention, the stator windings 21 areproduced from wire of a desired gauge, the wire comprising a singlestrand copper conductor pre-coated with insulation. The insulation mustbe rated to withstand 200° C. In this embodiment, Phelps DodgeIndustries GP/MR-200® brand wire or equivalent may be used. In thepreferred embodiment, the wire gauge typically will be AWG-13 orsmaller.

The wires are preshaped by methods known in the art into the desiredstator winding configuration. For example, the wires may be shaped toform a complete multiple phase stator winding, or may be shaped to formseparate single phase stator windings which subsequently may be combinedinto a multiple phase configuration, if the desired application sorequires. Because a stator produced according to the present inventionwill have a smaller radius and shorter axial length than a statorproduced by prior art methods, less wire is used in the manufacture of astator according to the present invention than in prior art methods.This reduces the cost of the finished stator, and also reduces the massof the finished stator.

During the compaction step of the method of present invention (discussedin more detail hereinafter), windings 21 will be exposed to compactionpressures above the pressure at which the common wire insulationpolymers will begin to flow. If the insulation fails during thecompaction process exposing the center conductor, the electricalintegrity of the stator will be compromised and the stator must bescrapped. To reduce insulation stresses during compaction, and toimprove the insulation packing factor, the plurality of wires comprisingthe active portion of windings 21 should be substantially parallel toeach other. In addition, crossovers of individual wires in the end turnregion of stator 20 should be minimized. Windings 21 may be taped toreduce the chance of movement during handling and compaction, but thisis not required.

The wires in the finished stator must bond to one another and to thepowdered iron. In one embodiment of the present invention, this willoccur due to changes to the surface of the wire insulation polymer underthe pressures and temperatures achieved during compaction. In anotherembodiment, a binder coating of a type known in the art, such as PhelpsDodge Industries BONDEZE® M, may be applied to the insulation coating topromote the bonding between the wires, and between the wires and thepowdered iron. Alternatively, a thermoset resin, such as a “B” stageepoxy, may be applied. The thermoset resin must be of a type which doesnot set before compaction.

After windings 21 are shaped into the desired configuration, they areplaced in a compaction die cavity. The compaction die cavity is sized toreceive windings 21 and the amount of powdered magnetic materialrequired to produce a finished stator of the desired dimensions. Afterwindings 21 are in place within the compaction die cavity, the remainingvolume of the compaction die cavity is filled with powdered magneticmaterial.

The powdered magnetic material fill step of this embodiment of thepresent will result in a powdered magnetic material fill density ofabout 50% before compaction. To match the powdered magnetic material“shrinkage” during compaction, windings 21 should be produced withinitial wire packing factor of about 50%. In another embodiment of thepresent invention, a higher packing factor, such as, for example, 75%may be used, provided the wire insulation is of a type that canwithstand powdered magnetic material flow past it during compaction. Inother embodiments of the present invention, flow enhancers can be addedto the powdered magnetic material or the wires can be lubricated toreduce the likelihood of insulation damage during compaction of thepowdered magnetic material.

In one embodiment of the present invention, the powdered magneticmaterial is “composite iron.” Composite iron is comprised of iron alloypowder coated with a thermoplastic material or a thermoset material. Theparticle sizes of the iron alloy powder used in one embodiment of thepresent invention range from about 44 to about 250 microns. A smallpercentage of this iron alloy powder may have a particle size as smallas about 10 microns. The iron alloy powder used in this embodiment isabout 99.7% Fe, 0.003% C, 0.0005% N, 0.006% S, and 0.004% P by weight.

The iron alloy powder particles are either coated with a thermoplasticmaterial or a thermoset material, or the thermoplastic material or athermoset material is admixed with the iron alloy powder to form thebasic composite iron. The magnetic, thermal, and mechanical propertiesof the composite iron are interrelated through iron alloying, particlemorphology, particle size, thermoplastic material or thermoset materialtype, and the ratio by weight of the iron alloy to the thermoplasticmaterial or to the thermoset material.

The thermoplastic material in one embodiment of the present inventionmay be an amorphous thermoplastic polyethermide resin, an example ofwhich is a General Electric ULTEM® resin. The thermoplastic materialused in this embodiment may be replaced in other embodiments of thepresent invention by a thermoset material, such as polyethersulfone(“PES”) and/or PPS. Other commercially available materials which possesssimilar properties may be used.

To prepare the composite iron, the particles of iron alloy powder inthis embodiment of the present invention are coated with a thin layer ofthe thermoplastic material. If the thermoplastic material is not liquidunder ambient conditions, it must be liquefied. In one embodiment, thethermoplastic material is mixed with a solvent to provide a liquidmaterial. In another embodiment, the thermoplastic material is heateduntil liquefaction.

In one embodiment of the present invention, the iron alloy powder iscoated with the thermoplastic material when it is blown by compressedair up through a vertical tube and, at the same time, the liquidthermoplastic material is sprayed on the blown iron alloy powder to coatthe powder. The coated iron alloy powder falls outside the tube and isdirected back into an inlet of the tube where it is blown up again andcoated again. After a number of passes through the tube, the particlesof iron alloy powder are coated to the extent desired. If a solvent hasbeen used to liquefy the thermoplastic material, the solvent evaporatesor is recovered during this process.

To provide output performance, power, and torque comparable to a statorcore structure formed from steel laminations, the proportions of ironalloy powder and thermoplastic material in the finished stator coreshould fall within certain ranges. In one embodiment of the presentinvention, the stator core structure should be approximately between 95to 99.9% iron alloy powder by weight, with the remaining weightcomprised substantially by the thermoplastic material. Since about 1% byweight of thermoplastic material used in this embodiment is equal toabout 4% by volume, a stator core structure of this embodiment that isapproximately 99% iron alloy powder by weight and approximately 1%thermoplastic by weight, would be in the range of approximately 96% ironalloy powder by volume and approximately 4% thermoplastic material byvolume.

In the final molded state, the thermoplastic material is bonded to theouter surface of each iron alloy powder particle so that the particleswithin the finished stator core are insulated from each other by thinlayers of thermoplastic material. In addition, the thermoplasticmaterial surrounding each iron alloy particle will bond to thethermoplastic material surrounding adjacent iron alloy particles,creating the stator core structure. Those skilled in the art willappreciate that, since the thermoplastic material separates theparticles, there are gaps formed between the particles. These gaps actlike air gaps, since the thermoplastic material separating the particleshas about the same magnetic permeability as air. This gap effectincreases resistivity and, consequently, reduces eddy current loses.

To produce the finished stator core 20, the powdered magnetic materialsurrounding windings 21 must be compacted within the compaction diecavity to form the powdered magnetic material into the solid stator core22. A stator core produced with powdered magnetic material compacted byconventional means, such as the axial means or the isostatic means usedto produce the rotor disclosed in U.S. Pat. No. 5,536,985, can achievedensities of only approximately 95% of full density under optimalcircumstances. The permeability and saturation flux density of a statorcore produced from conventionally compacted powdered magnetic materialof this density both are significantly lower than that of steellaminations. A stator core produced from conventionally compactedpowdered magnetic material also is significantly weaker than a statorcore produced from steel laminations, often resulting in damage duringhandling. For a stator core produced from compacted powdered magneticmaterial to be a practical alternative to a stator core produced fromsteel laminations, higher densities must be achieved. The magneticproperties of a stator core fabricated from compacted powdered magneticmaterial improve with density. The mechanical and thermal performance ofcompacted powdered magnetic material increases with density.

To achieve an optimal finished stator, the powdered magnetic materialsurrounding windings 21 must be compacted in a way which does not resultin buckling and distorted geometry in the active portion of windings 21.If the geometry of the preformed windings 21 is altered, theelectromagnetic characteristics of stator 20 will be changed. Compactingthe powdered magnetic material by to the axial compaction method or theisostatic method disclosed in U.S. Pat. No. 5,536,985 will cause theactive portion of windings 21 to buckle and distort. To prevent bucklingand distortion in the active portion of windings 21, the powderedmagnetic material surrounding windings 21 must be compacted radially.

In one embodiment of the present invention, the necessary stator coredensity and undisturbed windings geometry may be achieved by radiallycompacting the powdered magnetic material using the dynamic magneticcompaction (“DMC”) process disclosed by Chelluri et al. in U.S. Pat. No.5,405,574, and in U.S. Pat. No. 5,689,797. DMC provides two advantagesover prior art compaction techniques. First, powdered metal structurescompacted by DMC are more dense than powdered metal structures compactedby other compaction techniques known in the art. DMC can produce anearly fully dense powdered magnetic material stator core. High densityis required to withstand the mechanical and thermal stresses, and toprovide the magnetic performance necessary to make powdered magneticmaterial a viable substitute for steel laminations in statorfabrication. The second advantage provided by DMC is that DMC enablesradial compaction of the powder magnetic material. Prior art devicesemploying embedded windings used axial compaction or isostaticcompaction, neither of which is suitable for the present invention. Bothwould result in an unacceptable distortion of the stator windings duringcompaction of the powdered magnetic material. An alternate means ofradial compaction may be used if the means is capable of producing apowdered magnetic material stator core of the required density.

The most critical areas of magnetic performance are the teeth 23 ofstator 20. Teeth 23 also are high mechanical and thermal stress areas,so mechanical and thermal stress resistance must be maximized in teeth23. The radial compaction inherent in DMC promotes powdered magneticmaterial powder flow into the teeth during compaction to maximizedensification, thereby improving magnetic performance, and mechanicaland thermal stress resistance.

FIG. 3 shows a cross-sectional view of a typical DMC apparatus. FIG. 4shows a top view of the DMC apparatus of FIG. 3, after compaction iscomplete. In FIG. 3 and FIG. 4, a compaction die cavity is formed by anelectrically conductive outer compaction container 30, an inner die core31, a die cavity floor 35, and a removable die cavity lid 36. A coil 32surrounds outer compaction container 30. Inner die core 31 is solid,cylindrical and has an outer diameter equal to the desired innerdiameter of the stator. The compaction die cavity is sized to receivewindings 21 and a predetermined amount of powdered magnetic materialparticles 33. The DMC apparatus also possesses a means for permittingthe release of air from the compaction die cavity during compaction (notillustrated in FIG. 3 or FIG. 4). This air release means is sized toprevent the release of powdered magnetic material during compaction.

In operation, die cavity lid 36 is removed and windings 21 are placed inthe desired position within the compaction die cavity. The remainingvolume of the compaction die cavity is filled with powdered magneticmaterial particles 33. Die cavity lid 36 is replaced. A short pulse(typically less than 1 ms) of a very large electrical current(approximately 40,000-60,000 amps) is conducted through coil 32,producing a high magnetic field (typically 30-40 T), which in turnproduces strong magnetic forces (represented by arrows 34) in the radialdirection of the coil 32 and inward on the electrically conductive outercompaction container 30. Outer compaction container 30 is collapsedradially inwardly, compacting the powdered magnetic material particles33 into a solid powdered magnetic material structure, and embeddingwindings 21 within the solid powdered magnetic material structure.

Inner die core 31, die cavity floor 35, and removable die cavity lid 36must be able to withstand the compaction forces generated by coil 32without distorting or changing position relative to each other. Themagnetic forces produced by coil 32 force the powdered magnetic materialparticles 33 together within the compaction die cavity, forming a verydense solid powdered magnetic material structure. The windings 21 maymove slightly inward toward the inner die core 31 because of themagnetic forces 34, but this movement may be anticipated and compensatedfor in the design of the DMC apparatus and windings 21, and in theplacement of the windings 21 within the compaction die cavity. Aftercompaction, outer compaction container 30 is removed destructively,revealing the finished stator 20.

Those skilled in the art will be appreciate that the above describedmethods of manufacture are embodied by the process comprising the stepsof (a) shaping an insulated conductor of a desired gauge into statorwindings 21 of a desired configuration; (b) placing stator windings 21into a compaction die cavity; (c) filling the remaining volume of thecompaction die cavity with a predetermined quantity of powdered magneticmaterial particles 33; and (d) radially compacting the powdered magneticmaterial particles 33 to cause the powdered magnetic material particles33 to bind together into a solid powdered magnetic material structurewith windings 21 embedded therein.

FIG. 5 shows a perspective partially fragmentary view of a statoraccording to the present invention. In FIG. 5, stator 50 is shown havinga first flat axial end surface 51, a second flat axial end surface 52,an outer annular surface 53, an inner annular surface 54, and aplurality of electrical connector leads 55. The missing fragment revealsa conceptual representation of stator windings 56. The stator windings,including the end turns, are fully enclosed within the core of stator50. The only exposed electrical conductors are electrical connectorleads 55, which pass through the surface of stator 50 and electricallyconnect to the embedded stator windings 56. In preferred embodiments,the electrical connector leads 55 may pass through either the firstaxial end surface 51, the second axial end surface 52, or the outerannular surface 53.

For a stator slot carrying a given total current, the present inventionenables the conductor packing factor to be increased from about 30%,which was typical of the prior art, to greater than 90%. The conductorpacking factor is increased by reducing the size of the stator slot. Theair is virtually eliminated from the stator slot. Several advantages ofthe present invention accrue from the reduced stator slot size.

First, the reduced stator slot size enables the diameter of the innersurface 54 and the diameter of the outer surface 53 to be reducedwithout affecting the magnetic loading. Reducing the diameter of theinner surface 54 of the stator enables the rotor radius to be reducedcorrespondingly. For a given total current and a given magnetic loading,a smaller rotor radius results in an increase in power density. It maybe necessary to increase the rotor's rotational speed to maintain arotor tip speed adequate to sustain the increase in power density.

Second, the reduced stator slot size results in a reduced stator slotpitch. For a given total current, a smaller slot pitch results in anincrease in electrical loading. For a given magnetic loading, anincrease in electrical loading results in an increase in torque densityand in power density.

In prior art stators the heat generated by electrical resistance in thewindings is primarily conducted to the end turns and rejected to the airfrom the end turns via either natural or forced convection. The lowconductor packing factor and use of non-reactive, non-conductivematerial to fill the stator slots essentially prevents thermalconduction to the stator core. In a stator according to the presentinvention, the conductor packing factor is higher and there is nomaterial insulating the conductors from the stator core. The completewindings, including end turns, are embedded in the stator core. All theresistive losses are conducted through the core and rejected from thesurfaces of the stator. In an alternate embodiment, the thermalconductivity of the stator core may be enhanced by the addition of highthermal conductivity carbon fibers to the composite iron beforecompaction.

Those skilled in the art will be appreciate that the foregoing methodwill produce a stator containing embedded windings which is suitable foruse in many applications, such as, for example, in vehicular alternatorapplications. It will be further appreciated by those skilled in the artthat a stator according to the present invention will produce outputperformance, power, and torque equivalent to that produced by a statorfabricated from slotted steel laminations, but with smaller overalldimensions and lower mass. Production waste is reduced in the presentinvention by fabricating the stator core from powdered magnetic materialinstead of steel laminations. Production cost also is reduced in thepresent invention by reducing production waste, and by reducing thematerial handling required during stator fabrication.

While this invention has been described as having a preferred design,the present invention can be further modified within the scope andspirit of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

We claim:
 1. A method for fabricating a electromagnetic devicecomprising the steps of: shaping an insulated electrical conductor intoa predetermined configuration of windings for an electromagnetic device;placing the shaped windings into a compaction die cavity dimensioned andshaped to form the exterior of the electromagnetic device; filling theremaining volume of said compaction die cavity with a quantity ofpowdered magnetic material capable of being bound together to form asolid structure upon compaction so that said windings are surroundedwith said powdered magnetic material; and radially compacting saidpowdered magnetic material within said compaction die cavity withsufficient force to cause said powdered magnetic material to bindtogether into a solid magnetic structure with said shaped windingsembedded therein.
 2. The method of claim 1, wherein said electromagneticdevice is a stator.
 3. The method of claim 1, wherein the step ofradially compacting said powdered magnetic material within saidcompaction die cavity is accomplished by dynamic magnetic compaction. 4.The method of claim 1, wherein said powdered magnetic material iscomposite iron.
 5. The method of claim 4 wherein said composite ironcomprises an iron alloy material coated with a thermoplastic material.6. The method of claim 4 wherein said composite iron comprises an ironalloy material coated with a thermoset material.
 7. The method of claim4 wherein said composite iron comprises an iron alloy comprising about99.7 % Fe, 0.003% C, 0.0005% N, 0.006% S, and 0.004% P by weight.
 8. Themethod of claim 1, further comprising the step of coating said insulatedelectrical conductor with a binder coating to promote bonding betweensaid windings and said solid magnetic structure.
 9. The method of claim1, further comprising the step of taping said shaped windings prior toplacement in said compaction die cavity to retard movement of saidshaped windings during the radial compaction step.
 10. A electromagneticdevice comprising: one or more electrical conductors shaped intowindings of an electromagnetic device in a substantially cylindricalconfiguration, wherein each adjacent conductor of said windings iselectrically insulated from each other adjacent conductor; a cylindricalbody comprising a plurality of radially compacted powdered magneticmaterial particles bound together due to compaction, surrounding andsolidly enclosing said one or more conductors, said cylindrical bodybeing electrically insulated from said one or more conductors, saidcylindrical body circumscribed by a continuous substantially cylindricalinterior surface and a continuous substantially cylindrical exteriorsurface substantially concentric therewith, a first axial end surfaceconnected between one edge of said continuous substantially cylindricalinterior surface and one edge of said continuous substantiallycylindrical exterior surface, and a second axial end surface connectedbetween the other edge of said continuous substantially cylindricalinterior surface and the other edge of said continuous substantiallycylindrical exterior surface, said second axial end surface beingopposite to and substantially parallel to said first axial end surface;and a plurality of electrical connector leads beginning outside saidcylindrical body and passing through at least one of the surfaces ofsaid cylindrical body, said plurality of electrical connector leadselectrically connected to said one or more conductors.
 11. Theelectromagnetic device of claim 10 wherein said electrical conductorsare shaped into single phase stator windings.
 12. The electromagneticdevice of claim 10 wherein said electrical conductors are shaped intomultiple phase stator windings.
 13. An electromagnetic device as claimedin claim 10, wherein said powdered magnetic material is composite iron.14. An electromagnetic device as claimed in claim 13, wherein saidcomposite iron comprises an iron alloy material coated with athermoplastic material.
 15. An electromagnetic device as claimed inclaim 13, wherein said composite iron comprises an iron alloy materialcoated with a thermoset material.
 16. An electromagnetic device asclaimed in claim 13, wherein said composite iron comprises an iron alloycomprising about 99.7% Fe, 0.003% C, 0.0005% N, 0.006% S, and 0.004% Pby weight.
 17. An electromagnetic device as claimed in claim 10, whereinsaid electrical conductors are coated with a binder coating to promotebonding between the windings and said compacted magnetic particles. 18.An electromagnetic device as claimed in claim 10, wherein said powderedmagnetic material is radially compacted using dynamic magneticcompaction.
 19. In an electromagnetic device of the type comprising arotor and a stator, an improved stator comprising: one or moreelectrical conductors preformed into stator windings in a substantiallycylindrical configuration, wherein each adjacent conductor of saidwindings is electrically insulated from each other adjacent conductor ofsaid windings; a cylindrical body comprising a plurality of radiallycompacted powdered magnetic material particles bound together due tocompaction, surrounding and solidly enclosing said one or moreconductors, said cylindrical body being electrically insulated from saidone or more conductors; a plurality of electrical connector leadsbeginning outside said cylindrical body and passing through a surface ofsaid body, said electrical connector leads being electrically connectedto said one or more electrical conductors.
 20. An improved stator asclaimed in claim 19, wherein said cylindrical body is circumscribed by acontinuous substantially cylindrical interior surface and a continuoussubstantially cylindrical exterior surface substantially concentrictherewith, a first axial end surface connected between one edge of saidcontinuous substantially cylindrical interior surface and one edge ofsaid continuous substantially cylindrical exterior surface, and a secondaxial end surface connected between the other edge of said continuoussubstantially cylindrical interior surface and the other edge of saidcontinuous substantially cylindrical exterior surface, said second axialend surface being opposite to and substantially parallel to said firstaxial end surface.
 21. An improved stator as claimed in claim 19,wherein said electrical conductors are shaped into single phase statorwindings.
 22. An improved stator as claimed in claim 19, wherein saidelectrical conductors are shaped into a multiple phase stator windings.23. An improved stator as claimed in claim 19, wherein said electricalconductors are coated with a binder coating to promote bonding betweenthe windings and said compacted magnetic particles.
 24. An improvedstator as claimed in claim 19, wherein said powdered magnetic materialis radially compacted using dynamic magnetic compaction.
 25. An improvedstator as claimed in claim 19, wherein said powdered magnetic materialis composite iron.
 26. An improved stator as claimed in claim 25,wherein said composite iron comprises an iron alloy material coated witha thermoplastic material.
 27. An improved stator as claimed in claim 25,wherein said composite iron comprises an iron alloy material coated witha thermoset material.
 28. An improved stator as claimed in claim 25,wherein said composite iron comprises an iron alloy comprising about99.7% Fe, 0.003% C, 0.0005% N, 0.006% S, and 0.004% P by weight.